GCN Circular 10813

D. Grupe (PSU), M. Siegel (PSU), J. Gelbord (PSU),  P.A. Evans and
M.R. Goad (U. Leicester) report on behalf of the Swift team:

We report of the Swift UVOT and XRT detections of a bright source  
within the 5'
error radius of the SuperAGILE detected GRB 100528A (Del Monte et al.,
GCN Circ 10810) at a level of 2e-12 ergs/s/cm2 in the 0.3-10 keV band.
Swift started observing the field of GRB 100528A 42.5
ks after the AGILE detection and has observed it so far for 5 ks (two
orbits). At this point it is not obvious if the X-ray source is fading.
However, this X-ray source was not present during the ROSAT All-Sky
Survey. There is also an archival XMM data set that covers this
position (OBSID 0037990101, a 17ks exposure taken 2002-11-21).
There is no source detected at the position of the counter part.
Measuring the flux within a 30" radius of this position, the 4-sigma
upper limit of 5e-14 ergs/s/cm2 in the 0.2-12 keV band.  Most likely
the X-ray source found by the XRT is the X-ray afterglow of GRB 100528A.


Using 2497 s of XRT Photon Counting mode data and 2 UVOT
images, we find an astrometrically corrected X-ray position (using the
XRT-UVOT alignment and matching UVOT field sources to the USNO-B1
catalogue): RA, Dec =
311.14130, 27.80660 which is equivalent to:

RA (J2000):  20 44 33.92
Dec (J2000): +27 48 23.8

with an uncertainty of 2.1 arcsec (radius, 90% confidence).

This position may be improved as more data are received.  The latest
position can be viewed at http://www.swift.ac.uk/xrt_positions. Position
enhancement is described by Goad et al. (2007, A&A, 476, 1401) and Evans
et al. (2009, MNRAS, 397, 1177).

Using 4985 s of UVOT data take in the u, v and White filters,
we identify a faint uncataloged source within the XRT error circle
at a position in the white images of RA,DEC=311.14130, 27.80660 which
is equivalent to:

RA (J2000): 20 44 33.92
Dec (J2000): +27 48 23.8

with a 90%-confidence interval of about 0.60 arc sec. Preliminary
magnitudes using the  UVOT photometric system (Poole et al. 2008,
MNRAS, 383, 627) are:

FILTER     T_start(s)  T_stop    Exposure        Mag
=========================================================
white      43296       49881      1632     20.96+-0.13
v          44124       50777      1721     20.01+-0.35
u          42469       49036      1632     20.63+-0.24
=========================================================

The above magnitudes are not corrected for the Galactic extinction
corresponding to a reddening of E_{B-V} = 0.29 (Schlegel et al.,
1998, ApJS, 500, 525). The photometry is on the UVOT photometric
system described in Poole et al. (2008, MNRAS, 383, 627).



This circular is an official product of the Swift team.